Working Group I: The Scientific Basis

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Figure 2.6: Smoothed annual anomalies of global average sea surface temperature (°C) 1861 to 2000, relative to 1961 to 1990 (blue curve), night marine air temperature (green curve), and land-surface air temperature (red curve). The data are from UK Met Office and CRU analyses (adapted from Jones et al., 2001, and Parker et al., 1995). The smoothed curves were created using a 21-point binomial filter giving near-decadal averages. Also shown (inset) are the smoothed differences between the land-surface air and sea surface temperature anomalies.

Figure 2.7: Smoothed annual anomalies of combined land-surface air and sea surface temperatures (°C), 1861 to 2000, relative to 1961 to 1990, for (a) Northern Hemisphere; (b) Southern Hemisphere; and (c) Globe. The smoothed curves were created using a 21-point binomial filter giving near-decadal averages. Optimally averaged anomalies (Folland et al., 2001) solid curves; standard area weighted anomalies (adapted from Jones et al., 2001) dashed curves. Also shown are the unsmoothed optimum averages red bars, and twice their standard errors width denoted by black I. Note that optimum averages for the Southern Hemisphere are a little warmer before 1940, when the data are sparse, than the area-weighted averages. However, the two types of averaging give similar results in the Northern Hemisphere.

Figure 2.8: Smoothed annual anomalies of global combined land-surface air and sea surface temperatures (°C), 1861 to 2000, relative to 1961 to 1990, and twice their standard errors. The smoothed curves and shaded areas were created using a 21-point binomial filter giving near-decadal averages, with appropriate errors. Optimally averaged anomalies and uncertainties (Folland et al., 2001) solid curve and dark shading; standard area weighted anomalies and uncertainties (adapted from Jones et al., 1997b, 2001) dashed curve and light shading. Note that uncertainties decrease after 1941 due to the cessation of uncertainties due to bias corrections in sea surface temperature. On the other hand, uncertainties due to urbanisation of the land component, assessed as zero in 1900, continue to increase after 1941 to a maximum in 2000.

Figure 2.6 summarises the relative changes of UKMO SST, UKMO NMAT and CRU land-surface air temperature. The greater warming of the land in recent years is clear, but otherwise all three curves have a generally similar shape except that modest cooling of NMAT in the late 19th century is not seen in the SST data as noted for Figure 2.5. The relative coldness of the land around 1885 to 1895 comes from the Northern Hemisphere continental interiors, particularly in winter, as global coastal land air temperature and adjacent SST anomalies agree well at this time (Parker et al., 1995), confirmed by the Jones et al. (2001) data. Note that there are some systematic compensating differences between the land and SST in the late 19th century in both hemispheres (not shown). The CRU land data are generally about 0.1 to 0.2°C colder in the Northern Hemisphere except at the beginning of the record (early 1860s), when they agree, and rather colder than this in 1885 to 1890. The opposite is seen in the Southern Hemisphere before 1885 when SST is generally 0.1 to 0.2°C colder and 0.3°C colder around 1875. Overall the SST data are less variable in each hemisphere in these rather poorly observed periods. The Southern Hemisphere land temperature at this time can actually represent a very small observed area of the hemisphere while the SST data, though sparse, are generally considerably more widespread. The sharp cooling in SST around 1903/4 in Figures 2.5 and 2.6, seen in the land as well as the two ocean surface data sets, was discussed for the North Atlantic and Indian Oceans by Helland-Hansen and Nansen (1920) not long after the event. The reduced warming of the NMAT in the last decade reflects differences in the Southern Hemisphere discussed above. Slightly greater warming of the global ocean than the global land in 1910 to 1945 (seen in Table 2.1) is within the uncertainties of either data set, as a slightly slower warming of the ocean might be expected on physical grounds.

Figures 2.7a to c show annual time-series of anomalies of combined land-surface air temperature and SST for the hemispheres and globe since 1861, based on the latest CRU land air temperature data and the UKMO SST data. Jones et al. (2001) temperature data have been averaged by both a standard weighting method, used in the SAR, as shown by the dashed smoothed curves, and by an optimum averaging method (Shen et al., 1998; Folland et al., 2001) as shown by the bars and solid smoothed curves. The latter method uses the variance-covariance matrix instead of correlation functions (Kagan, 1997). The calculated uncertainties (twice the standard error) in the annual values are also shown (including the independent urbanisation and SST bias correction uncertainties). Optimum averaging gives less weight to areas of high data uncertainty than do ordinary averaging methods, and it takes much better account of data gaps. It also gives more weight to Antarctica, the great bulk of which (away from the Antarctic Peninsula) has warmed little in the last two decades (Comiso, 2000). Optimum averages can affect individual years markedly when data are sparse. Thus extra warmth of the warm year 1878 (strongly affected by the 1877/78 El Niño) in the Northern relative to the Southern Hemisphere in the area weighted average (not shown) disappears when optimum averages are used. In the Northern Hemisphere, the optimum averages are little different from area weighted averages, but they are consistently warmer in the sparsely sampled Southern Hemisphere before 1940, often by more than one tenth of a degree. The overall effect on global temperature is small, however (Figure 2.7c)

The five warmest global optimally averaged years since the beginning of the record in 1861 all occurred in the 1990s with 1998 having the warmest anomaly (0.55°C). This year was significantly warmer than the second warmest year, 1995 (0.38°C), while 1999 was fourth warmest year, despite the strong La Niña event. The remarkably consistent monthly global warmth of 1998 is discussed in Karl et al. (2000).

Table 2.2 shows linear trends of the annual optimum averages, and twice their standard errors, for the globe and hemispheres using the restricted maximum likelihood method as in Table 2.1 and allowing for the annual uncertainties due to data gaps, urbanisation over land, and bias corrections to SST. Since 1861 the hemispheres have warmed by approximately the same amount. However both the earlier period of warming (1910 to 1945) and the more recent one (1976 to 1999) saw rates of warming about twice as great in the Northern Hemisphere. There was continued (non-significant) warming in the Southern Hemisphere, though at a reduced rate, in 1946 to 1975, which partially offset (non-significant) cooling in the Northern Hemisphere over the same period to give a (non-significant) 0.03°C cooling globally. The global trend from 1861 to 2000 can be cautiously interpreted as an equivalent linear warming of 0.61°C over the 140-year period, with a 95% confidence level uncertainty of ± 0.16°C. From 1901 an equivalent warming of 0.57°C has occurred, with an uncertainty of ± 0.17°C.

Figure 2.8 shows a smoothed optimally averaged annual global time-series with estimates of uncertainty at ± twice the standard error of the smoothed (near decadal) estimate. Note that the optimum average uncertainties increase in earlier years mainly because of the much larger data gaps. Also shown are uncertainties estimated by Jones et al. (1997b) using a different method centred on the Jones et al. (2001) land and sea surface temperature series. This series uses the average of anomalies from all available grid boxes, weighted according to grid box area. Therefore, in contrast to the Jones et al. (2001) global land-surface air temperature data, the global land and sea surface temperature data are not a simple average of the hemispheres. The optimally averaged uncertainties vary from about 15 to 65% less than those given by Jones et al. (1997b). This is reasonable as optimum averages have minimum variance amongst the range of unbiased estimates of the average. Not surprisingly, there is relatively little difference in the decadal averages themselves. However unlike the Jones et al. estimates of uncertainty, the optimum average also includes uncertainties in bias corrections to SST up to 1941 (Folland and Parker, 1995) and the uncertainties (as included in Figure 2.1) in the land data component that are due to urbanisation. Cessation of the SST component of uncertainty after 1941 is the reason for a lack of increase in uncertainties in the fairly poorly observed period 1942 to 1945. Uncertainties due to changes in thermometer screens are poorly known but could be 0.1°C globally in the 19th and early 20th centuries (Parker, 1994); they are not included here, but a preliminary analysis appears in Folland et al. (2001). For further discussion of changes in land and ocean surface temperature, see Jones et al. (1999a).

Referring back to Table 2.2 and including the second decimal place, our best estimate of the equivalent linear rate of global land and ocean surface warming between 1861 to 2000 is 0.044°C/decade, or a warming of 0.61 ± 0.16°C. Over the period 1901 to 2000, the equivalent values are 0.058°C/decade or a warming of 0.57 ± 0.17°C. These values include the modifying effects of the annual uncertainties. So we calculate that since the late 19th or the beginning of the 20th century, up to 2000, global warming has been 0.6 ± 0.2°C. This is 0.15°C more warming than the 0.3 to 0.6°C estimated more subjectively up to 1994 by the SAR. This relatively large increase is explained by the increase in temperature since the SAR was completed, improved methods of analysis and the fact that the SAR decided not update the value in the First Assessment Report, despite slight additional warming. The latter decision was likely to have been due to a cautious interpretation of overall uncertainties which had at that time to be subjectively assessed.

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